HANDS ON GUIDE

The Tesselator

The R600's Tesselator isn't a part of the DirectX 10 specification, but a by-product of ATi's work on the Xenos processor in the Xbox 360. It provides a cheap way to produce seemingly more detailed geometry and smoother, less obviously polygonal surfaces by taking the basic polygon mesh of a surface and subdividing it into a more detailed mesh, creating up to (roughly) 1,300 triangles for every one in the original. These triangles can then be processed through the vertex-assigned stream processors in the standard way. 'It's computationally incredibly cheap,' says ATi's Richard Huddy. 'Compared with trekking those 1,300 triangles through the engine in the regular way, it's now just over three times as fast.'

The benefit of this is that you only have to call one triangle from memory, so there are advantages to using the Tesselator in low-end HD 2600-series or HD 2400-series cards, which have 256MB or less of memory and tighter bandwidth. 'That's a big surprise,' says Huddy, tongue in cheek, 'because we tend to reserve the coolest benefits for those who spend the most money, but here you actually get the full-on benefits - and you'll feel it more - on low-end hardware.'

Of course, there is a problem: with the Tesselator not being part of the official DirectX 10 spec, not every developer or game will necessarily support it. ATi's hopes rest on cross-platform Xbox 360/PC development. In Huddy's words, the Tesselator 'is cut and paste from the Xbox 360 - it's basically the same piece of hardware - so we have high expectations of Xbox 360 ports making very straightforward use of it. We're working with developers who aren't working on 360 ports to make sure they use it too, but it's a no-brainer for anyone performing Xbox 360 work.'

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